This invention relates to a variable capacitance circuit capable of varying electrostatic capacitance values in accordance with an application of a direct current voltage as a control voltage, a variable capacitance device, a resonance circuit using the same circuit, and a communication apparatus using the same circuit.
Various technologies have been developed in the art of near field communication (NFC) system, in which non-contact data transmission is performed between IC cards such as, e.g., railroad tickets (e.g., Suica (registered trademark)) as well as electronic moneys, e.g., FeliCa (registered trademark) and a reader-writer device.
In such a near field communication system, a receiver side has a resonance circuit made of an antenna coil and a capacitor, and does communication and transmits electric power upon resonating a signal outputted from a transmitter side. Good effective transmission can be done if the transmission frequency and the resonance frequency on the receiver side are the same to each other. In fact, there are some problems such as deviations of constants of resonance capacitors and deviations of resonance frequency on the power receiver side due to such as a correlative position to the power transmitter side.
For this problem, for example, Patent Document No. 1 (see, below) discloses a method adjusting the resonance frequency discretely by switching the resonance capacitors in the resonance circuit. That is, with Patent Document No. 1, disclosed is the method for adjusting resonance frequency in which plural capacitors forming the resonance circuit are connected in parallel and turned on and off by the switches respectively to change the capacitance. Adjusting accuracy, however, may be limited because the change of capacitance is stepwise.
To the contrary, for example, Patent Document No. 2 proposes a method for adjusting the resonance frequency in an analog manner in which the resonance capacitor is formed of variable capacitance elements whose control voltages are applied with fixed steps. That is, the variable capacitance elements can change the resonance frequency because the capacitance changes when applied with the direct current voltage (control voltage).
More specifically, the Patent Document No. 2 shows that the resonance frequency can be adjusted easily as no need for peak detection by utilizing phase differences of signals corresponding to input and output of the resonance antenna as monitoring signals for adjustment and discloses a method seeking the optimum value upon increasing the adjustment voltage in a stepwise manner. Because the method increasing the control voltage in the stepwise manner in fixing the step width takes more time for adjustment, a method shortening the adjusting time by changing the step width has been known. Generally, it is said that a half dividing method, in which the control voltage range is narrowed in making the range half sequentially at each step, has a less adjusting step number for searching the entire range of the control voltage than that of a sequential seeking method.
FIG. 9 shows response characteristics of the variable capacitance element, and it is described. In FIG. 9, the control voltage changing the capacitance of the variable capacitance element and the consequence that the phase difference from the capacitance change is converted into the voltage are compared. According to FIG. 9, it turns out that the phase difference voltage increases or decreases exponentially gradually even where the control voltage changes stepwisely. The response at which the control voltage is changed from 3V to 0V is faster than that at which the control changed from 0V to 3V. It is to be noted that the response time is 300 to 400 milliseconds in this example.
Patent Document No. 1: Japanese Patent Application Publication No. 2008-160312
Patent Document No. 2: Japanese Patent Application Publication No. 2012-099968